Software Defined Networking (SDN) is an approach to computer networking that allows network administrators to manage network services through decoupling the system that makes decisions about where traffic is sent (the control plane) from the underlying systems that forward traffic to the selected destination (the transport and data plane).
SDN can allow for centralized control of routing and other control plane functions. This can allow for lower cost networking nodes or, at least, nodes that do not need to know the network topology in order to route data. A centralized controller also allows for a better picture of the network. This more complete picture can allow for root causes of networking problems to be addressed even if the nodes affected by the solution are removed from the problem.
Signaling control networks (SCNs) designed for a distributed control plane may not have sufficient capacity to handle some centrally controlled SDN features. Notably, an optical network control plane may be regarded as an example distributed control plane.